The subject matter of the invention is a filling level sensor with a float which is fastened to a lever arm and at least one contact which is arranged on the lever arm and is in connection with a slider track formed as a thick-film resistor.
Such filling level sensors for a fuel tank of a motor vehicle have long been the known state of the art. The thick film is in this case applied to a substrate, which is often a ceramic. The actual thick film comprises one or more metals. It is known to use silver for this, on account of its good conductivity and corrosion resistance. When such a thick-film resistor is used in a filling level sensor in a fuel tank, it has been found that, in spite of its good corrosion resistance, silver is attacked by fuels. In particular, some aggressive constituents in certain fuels react with silver in such a way as to cause deposits on the surface of the thick-film resistor over the operating time of the filling level sensor. These deposits are characterized by a changed conductivity. As a consequence, the deposits lead to changes in the measured resistance values that are contrary to the actual conditions in the fuel tank. The filling level sensor consequently no longer operates accurately, or in the worst case the filling level sensor fails.
It is also known to use gold or platinum as a material in thick-film resistors to avoid this disadvantage. Such thick-film resistors do indeed have a higher corrosion resistance. Apart from the higher costs for these materials, the adhesive strength of such thick films is inadequate. Moreover, thick films of gold or platinum have a poor soldering behavior. In order nevertheless to achieve adequate contact stability, they have disproportionately large soldering areas for the terminal contacts. This configuration with the associated high costs has previously been accepted, and regarded as the technical optimum.
The present invention is based on the object of providing a filling level sensor with a thick-film resistor which has a high corrosion resistance with respect to fuels and ensures good adhesive strength on a substrate.